Absence of oxygen induced marked changes in the mitochondrial system of the obligate aerobic yeast Rhodotorula gracilis. The number of mitochondrial profiles diminished progressively in proportion to increase in time of anaerobic growth. After 8 h of anaerobiosis, when the cell survival rate was still high (95%) almost all cells showed a single broad mitochondrial profile. Protein and RNA synthesis were blocked, and cytochrome levels a3, a and c remained unchanged. When oxygen was again introduced, growth continued and the cell promptly recovered from all the changes observed under anaerobic conditions. The morphological changes in the mitochondrial system were interpreted as a change of form in what was probably a single mitochondrion.